The present invention relates generally to ceramic composites of ceramic matrices reinforced with about 1 to 60 vol. % of elongated monocrystalline silicon carbide whiskers homogeneously dispersed therein, and more particularly to such composites wherein the whiskers are modified or beneficated to provide a relatively uniform whisker size distribution and the surface chemistry is modified to inhibit chemical bonding of the whiskers to the matrix material. This invention was made as a result of work under a contract DE-AC05-840R21400 between Martin Marietta Energy Systems, Inc., and the U.S. Department of Energy.
Recent developments in the strengthening or toughening of ceramic materials for reducing cracking and other deleterious structural problems associated with ceramic materials have enhanced their utilization in ambient temperature as well as high-temperature applications such as in the fabrication of heat engine components, bearings, valves, cutting tools and the like. For example, as described in U.S. Pat. No. 4,543,345 which issued Sept. 24, 1985, ceramic composites have been considerably toughened with respect to cracking by reinforcing the ceramic matrices with elongated monocrystalline silicon carbide whiskers in concentrations of about 10 to 60 vol. %. The ceramic composites are formed by mixing the silicon carbide whiskers with fine ceramic powders such as alumina, mullite, boron carbide, silicon nitride, zirconia, or yttria. These mixtures are blended to provide a homogeneous dispersion of the silicon carbide whiskers therein and are then hot-pressed at temperatures in the range of about 1600.degree. to 1950.degree. C. at pressures in the range of about 28 to 70 MPa for a duration of up to about 2.5 hours to provide SiC whisker-reinforced ceramic composites. The composites are characterized by a significant increase in fracture toughness over that afforded by the matrix material without the inclusion of the silicon carbide whiskers. Alternatively, the silicon carbide whiskers may be blended with the ceramic powders and formed into more intricate shapes than obtainable by hot-pressing by utilizing a pressureless sintering technique as described in U.S. Pat. No. 4,652,413 which issued Mar. 24, 1987. The SiC whiskers may also be incorporated into mullite and alumina matrices toughened with zirconia additions as described in U.S. Pat. No. 4,657,877 which issued Apr. 14, 1987. These patents are incorporated herein by reference.
The silicon carbide whiskers which are homogeneously dispersed within the ceramic matrix material described in the aforementioned patents provide the toughening for the ceramic composite by being contained in the ceramic material with a relatively weak chemical bond at the interface between. Thus, when a SiC whisker-reinforced ceramic composite is subjected to a crack-inducing stress which is sufficient to cause a crack to propagate into the composite, the whiskers effectively act as "reinforcing bars" in that when the crack propagates into the ceramic matrix material and the crack encounters a whisker the weak chemical interface debonds and the crack is forced to change directions and break around the whiskers. The whiskers also "bridge" the crack to deter further crack propagation. Alternatively, if the whisker is excessively chemically bonded to the ceramic matrix material to inhibit "crack-bridging" or whisker pullout, the whiskers will simply break as the crack propagates through the ceramic matrix material since sufficient bonding occurs at the whisker-matrix interface so that the whisker behaves essentially similarly to that of the monolithic ceramic matrix.
Silicon carbide whiskers of essentially a monocrystalline microstructure which are suitable for use in the fabrication of whisker-reinforced ceramic material as described in the aforementioned patents, are presently available from numerous manufacturers in wide variations of size with these whiskers being of a diameter in the range of about 0.1 to about 10 micrometers and a length of about 10 to 200 micrometers. These whiskers have a length to diameter ratio of at least about 10. It has been recently discovered that these commercially available silicon carbide whiskers as supplied by the various manufacturers, are subject to considerable variations in physical and chemical properties with these variations occuring even in batches supplied by the same manufacturer which may adversely affect their utilization for reinforcing ceramic matrices for crack toughening purposes as described above. Further, SiC whiskers, even when of the same dimensions and supplied by the same manufacturer in different lots, have been found to introduce significant differences in the fracture toughness of ceramic composites.
It has been found that these problems are partially due to the fact that each lot or batch of whiskers supplied by any of the various manufacturers contains a considerable percentage of whiskers which have structural anomalies such as holes in the sides, hollow sections, significant diameter variations, length to diameter ratios less than about 10, or defect inclusions, as well as other structural features which detract from the strength of the individual whisker when utilized as a "reinforcing bar" in a ceramic composite. The concentration of the whiskers which are incapable of providing satisfactory ceramic fracture toughening varies from batch-to-batch but is normally in the range of about 1 to 25 wt. % of each batch of whiskers presently provided by the various manufacturers. Included with the defect whiskers is extraneous debris, such as rice hulls and SiC particulates from the whisker manufacturing process, that is associated with the whiskers.
An even more critical problem attendant with the presently supplied SiC whiskers is that the surfaces of the whiskers contain considerable concentrations of impurities of non-silicon carbide metals and compounds which are capable of chemically reacting with the ceramic material forming the matrix for effecting a relatively tenacious chemical bond between the whiskers and the ceramic material. This chemical bonding inhibits the necessary "de-bonding" between the whiskers and the ceramic material needed for providing the whisker crack-bridging, whisker pullout and crack deflecting properties in the ceramic matrix. Of these surface "impurities" it is believed that the most troublesome with respect to providing a chemical bond with the ceramic matrix material are silicon dioxide (SiO.sub.2) and calcium oxide (CaO) since these compounds are glassforming compounds and are usually present in the largest quantities and fuse the whiskers to the ceramic matrix when heated to a temperature greater than about 800.degree. C. which normally occurs during the fabrication of the composites such as described in the aforementioned patents. These glass-forming compounds have been found to be present in significant concentrations on the surface of the whiskers presently provided by all of the commercial suppliers with these concentrations varying from manufacturer to manufacturer and from batch from the same manufacturer. Based upon the analyses of the silicon carbide whiskers presently available from each of the several commercial suppliers, it is expected that silicon dioxide on the surface or near surface regions of the whiskers will be in the order of a silicon dioxide-to-silicon carbide ratio in the range of about 0.1 to 1.0. The concentration of calcium oxide is in the range of about 0.5 to 2 wt. % of the whisker.
In addition to the aforementioned impurities of silicon dioxide and calcium or calcium oxide on the surface or near surface regions of the silicon carbide whiskers it has been found that the commercially available whiskers also contain relatively small concentrations of boron, magnesium, iron, chromium, cobalt, manganese, nickel, sodium, zinc, and copper, and oxide-containing compounds thereof which are also capable of effecting a chemical bonding relationship with the ceramic matrix material during the fabrication of the composites.
The presence of the physical anomalies in each batch of the whiskers and the surface impurities on the surface or near surface regions of commercially available silicon carbide whiskers have been found by applicants to be the primary mechanisms responsible for the production of silicon carbide whisker-reinforced ceramics with heretofore essentially unexplainable large variations in fracture toughness even in similarly produced ceramic composites using the same whisker concentrations, sizes, and supplier. Therefore, it is believed that the manufacturers of silicon carbide whiskers are presently providing commercially available whiskers with large batch-to-batch differences in surface impurities and physical anomalies are apparently unaware of the problems that these impurities and physical anomalies cause in the manufacture and reproducibility of silicon carbide whisker-reinforced ceramic composites.
Some silicon carbide whiskers such as those obtained from rice hulls as described in the aforementioned patents and supplied by ARCO Metals, Silag, Operations, Green, S.C., and whiskers in a larger diameter range of about 0.5 to 5 .mu.m as prepared and supplied by Los Alamos National Laboratory at Los Alamos, N.M., have sometimes been found to possess a layer of free carbon on the surface of the whisker. It has been discovered by applicants that SiC whiskers with a layer of free carbon essentially covering exposed surface portions thereon did not become sufficiently chemically bonded to the matrix material so as to deter desired bridging or deflecting properties of the whiskers. This apparent lack of a "strong" chemical bond between the ceramic matrix material and the whiskers containing the layer of free carbon is believed to occur even if the surface or near regions of the whiskers contain silicon dioxide and/or calcium oxide in such concentrations that would, except for the presence of the layer of free carbon, effect a chemical bond of the whisker to the ceramic matrix sufficient to deter crack bridging. This layer of free carbon is believed to act as a barrier between the aforementioned glass-forming compounds and the ceramic matrix material to inhibit or detract from ceramic bonding reactions between the whisker and the ceramic material. It has also been determined that this layer of free carbon is not purposely provided by the manufacturer on the surface of the SiC whiskers but is merely a by product of the particular whisker manufacturing process since it has been found that the free carbon on the whisker surfaces varies in concentrations or thicknesses from batch-to-batch with some batches of whiskers essentially lacking a layer of free carbon in a sufficient thickness to inhibit excessive chemical bonding of the whiskers to the matrix material due to the presence of the aforementioned surface impurities on the whiskers.